Impact of systemic comorbidities and interaction patterns on mortality following major emergency abdominal surgery: A nationwide register-based cohort study.

BACKGROUND: Current risk stratification tools in emergency surgery generally treat comorbidities as independent risk factors, potentially underestimating mortality risk in patients with multimorbidity. This study aimed to evaluate the individual and combined effects of systemic comorbidities on mortality after major emergency abdominal surgery. METHODS: This nationwide register-based cohort study included all adult patients undergoing major emergency abdominal surgery in Denmark between 2002 and 2022. The primary outcome was 365-day all-cause mortality, further stratified into immediate (days 0-7), short-term (days 8-90), and long-term (days 91-365) mortality. Associations between systemic comorbidities and mortality were assessed using Kaplan-Meier analyses and multivariable logistic regression. Multivariable least absolute shrinkage and selection operator regression was used to identify interaction patterns between specific comorbidity combinations. RESULTS: A total of 61,400 patients were included. Mortality rates were 11% at 7 days, 24% at 90 days, and 31% at 365 days. Increasing comorbidity burden was associated with higher mortality across all postoperative phases (P<0.0001). Liver disease demonstrated the strongest association with immediate mortality [odds ratio (OR): 2.75; 95% CI: 2.19-3.43, P<0.0001], whereas metastatic malignant disease was most strongly associated with short-term (OR: 5.27; 95% CI: 4.86-5.71, P<0.0001) and long-term mortality (OR: 9.37; 95% CI: 8.48-10.36, P<0.0001). Several comorbidity combinations demonstrated interaction patterns consistent with excess mortality beyond isolated effects, particularly combinations involving cardiac disease, renal disease, liver disease, and cerebrovascular disease. CONCLUSIONS: Systemic comorbidities were strongly associated with mortality after major emergency abdominal surgery. Several comorbidity combinations demonstrated positive interaction patterns exceeding isolated effects. These findings suggest that risk stratification models assuming independence between comorbidities may underestimate mortality risk in high-risk patients undergoing major emergency abdominal surgery. LEVEL OF EVIDENCE: Prognostic and Epidemiological Study; Level III.